Model is solidified but appears full during 3D print
When using a 3D printer, which has hollows, its like trying to print a bridge with the tunnel part underneath. The printer cannot do this not without supports being printed too as the maximum angle support is around 45 degrees. The way to over come this is to tilt your model so that printer can put layers on top of one another. This might not be your problem of course but it is worth considering.

The other way is to print the support using a material that you can wash out afterwards. It would mean changing the material over once the supports are printed.

If you model's outside angles (travelling upwards from the base)are all within the 45 degrees then it should be ok. But I suspect that the printer might be trying to put down layers where there is nothing underneath it, so it prints the whole model filled in. I've had this problem myself. Had to think outside the box for this one :)